Magnetorheological formulations (abbreviation: MRF) are generally designated as formulations which change their rheological properties under the action of a magnetic field. They are generally suspensions of ferromagnetic, superparamagnetic or paramagnetic particles in a liquid.
If such a suspension is exposed to a magnetic field, its flow resistance increases. This is caused by the fact that, owing to their magnetic interaction, the dispersed magnetizable particles, for example iron particles, form chain-like structures along the magnetic field lines. During the shearing of an MRF, these structures are partly destroyed but form again. The rheological properties of a magnetorheological formulation in a magnetic field resemble the properties of a plastic body having a flow limit, i.e. it is necessary to apply a minimum shear stress in order to cause the magnetorheological formulation to flow.
High transmittable shear stresses are required for the use of magnetorheological formulations in controllable apparatuses, such as shock absorbers, clutches, brakes and other devices (for example haptic devices, crash absorbers, steer-by-wire steering systems, gear- and brake-by-wire systems, seals, retaining systems, prostheses, fitness devices and bearings).
Known applications of magnetorheological liquids are described, for example, in U.S. Pat. No. 5,547,049, in EP 1 016 806 B1 or in EP 1 025 373 B1.
Formulations according to the prior art which are known to the person skilled in the art use hydrocarbons, for example alkanes, alkenes, poly-α-olefins (PAO) or esters, polyesters, silicone oils, polyalkylene glycols or water as a base liquid. Carbonyl iron powder—spherical iron particles having a size of from 1 to 30 μm—is frequently used as the magnetic component, although particles of other alloys (WO 94/10691) or having an irregular form are also described (WO 04/044931 or US 2004/140447).
Good suitability for use of a magnetorheological formulation demands a low tendency to sedimentation with the magnetizable particles used in the liquid. If sediments occur, they must be capable of being easily stirred, i.e. easily redispersed, in order to avoid adversely influencing the functioning of the apparatuses in which the magnetorheological formulation is used. The formation of agglomerates and hard sediments which are no longer redispersible can be completely or partly overcome by using suitable dispersants. As a rule, polymers or surfactants are used for this purpose. U.S. Pat. No. 5,683,615 describes the use of thiophosphorus and/or thiocarbamate compounds as dispersants for magnetizable particles for improving the colloid stability. US 2004/0084651 describes oleates, naphthenates, sulfonates, phosphate esters, laurates, stearates, e.g. lithium hydroxystearate, stearic acid, glyceryl monooleate and fatty alcohols as dispersants. US 2002/0130305 mentions ethoxylated alkylamines, such as, for example, tallow fatty amine ethoxylate, as preferred surfactants. US 2003/0047705 claims ethoxylated and propoxylated alkylamines.
In addition, the known magnetorheological formulations generally comprise a thixotropic agent which establishes a flow limit and thus counteracts sedimentation of the particles. The sediment hardness is reduced and the redispersibility of particles which have already settled out is facilitated by such additives. The prior art is the use of hydrophobically modified sheet silicates of the smectite type, particularly of the montmorillonite type (WO 01/03150 A1), a main constituent of bentonite, of silica gel or of disperse silica (U.S. Pat. No. 5,667,715) in nonpolar liquids. The use of carbon particles (U.S. Pat. No. 5,354,488) or of polyureas for this purpose (DE 196 54 461 A1) is also known.
Water-based magnetorheological formulations are described in U.S. Pat. No. 6,132,633 and comprise hydrophilic sheet silicates of the bentonite or hectorite type. Laponite, a synthetic sheet silicate similar to hectorite, is also mentioned for this intended use.
The transmittable shear stress of a magnetorheological formulation increases with the proportion by weight of the magnetizable particles. For individual applications, proportions by weight of the magnetizable particles of 90% or more are absolutely desirable. Strategies for maximizing the proportions by weight and hence the transmittable shear stress in a field relate to the fine tuning of the particle sizes, possibly the use of particle diameters of different magnitude (WO 97/15058). U.S. Pat. No. 5,667,715 relates to a mixture of large and small iron particles for maximizing the ratio of the transmittable shear stress in a magnetic field to the transmittable shear stress without a magnetic field. However, the close packing of magnetizable particles and the intrinsic viscosity or shear stress increasing with the degree of pigmentation constitute a limiting factor in every case. US patent application US 2006/0033068 therefore describes magnetorheological formulations having proportions of magnetizable particles which possess a special geometry. These particles in the form of lamellae, needles or cylinders or in an egg shape align themselves in the direction of flow of a liquid without influences of a magnetic field and therefore have a lower intrinsic viscosity with comparable maximum shear stress in a field in comparison with magnetorheological formulations comprising, for example, spherical particles.
A further strategy for maximizing the achievable shear stresses is the removal of troublesome impurities on the particle surfaces (WO 94/10694 or WO 95/28719) or the use of certain alloys (WO 94/10691).
It is known to the person skilled in the art that the polarity of a liquid present in the magnetorheological formulation plays a role in influencing the shear stresses achievable with the magnetorheological formulation in a magnetic field. Thus, poly-α-olefin-based magnetorheological formulations show lower shear stresses than silicone-based MR formulations or even aqueous systems. Polar additives to the liquid component of a magnetorheological formulation can contribute toward improved shear stresses.
Conventional polar liquids present in magnetorheological formulations, such as, for example, water or polyalkylene glycols, however, show an excessively high viscosity or solidification at low temperatures below −20° C. and are therefore eliminated for suitable magnetorheological formulations which have a high ratio of transmittable shear stress in a magnetic field to transmittable shear stress without a magnetic field.
Another, previously unsolved problem is the poor thermal stability of the liquid present in the magnetorheological formulation. Thus, a multiplicity of the known magnetorheological formulations which have a low viscosity at low temperatures and can therefore be used, for example, for the automotive sector is stable over a relatively long period only at temperatures up to 100° C., whereas there is no longer sufficient stability at higher temperatures up to 150° C., whether because of evaporation loss or because of chemical change in the liquid present in the magnetorheological formulation. In this context, “stable” is understood as meaning that the performance characteristics do not deteriorate as a result of thermal load. These are firstly the rheological properties, i.e. the flow behavior without a magnetic field and under the influence of a magnetic field. Secondly, the formulations should show no instabilities or inhomogeneities after being subjected to a thermal load for a relatively long time, such as agglomeration or pronounced sedimentation, for example with formation of hard sediments which are no longer redispersible, which is due inter alia to the partial or complete loss of function of the dispersant. Frequently, the liquids having a low viscosity at low temperatures and present in the magnetorheological formulation have too high a vapor pressure at temperatures above 150° C. Evaporation of liquid fractions at high operating temperatures and hence thickening of the magnetorheological formulation are the result. The known magnetorheological formulations comprising liquids which can be exposed to high operating temperatures of more than 170° C. without adversely affecting the life of the magnetorheological formulation are too highly viscous, solidify in amorphous form or crystallize at temperatures below −20° C. even without application of a magnetic field.
A disadvantage of the known magnetorheological formulations is that they frequently do not have the desired combination of properties for the respective fields of use. The individual components of the formulations—e.g. base liquid, viscosity modifier, magnetizable particles, dispersant, thickener, corrosion inhibitors and lubricant and others—should be tailored to one another for many applications so that, in spite of the high volume fractions of magnetizable particles, the usability of the formulation is ensured. This is understood as meaning the flowability of the formulations over a broad temperature range of, for example, from −40° C. to 200° C., as low a viscosity level as possible without the action of a magnetic field, as high a transmittable shear stress as possible in a magnetic field, little sedimentation of the magnetizable particles, little tendency to aggregation and easy redispersibility after sedimentation. Another important property is high stability of the magnetorheological formulation to energy inputs which results from use. The energy is input by high shearing with and without magnetic field and manifests itself in high fluid temperatures, abrasion and physical and chemical fluid changes.